Device for determining and/or monitoring a predetermined filling level in a container

ABSTRACT

A device for establishing and/or monitoring a predetermined fill level in a container is provided, to indicate the fill level in a container and exhibiting a best possible, optimum fit for an application, which device includes: a mechanical oscillatory structure placed at the height of the predetermined fill level, which structure exhibits a membrane and two mutually separated oscillation bars formed thereon, an electromechanical transducer, which in operation excites the oscillatory structure to oscillate with oscillations, such that the oscillation bars execute oscillations perpendicular to their longitudinal axis, a receiver- and evaluation-unit, which uses the oscillations to establish and/or monitor whether the predetermined fill level has been reached, or not, in which the oscillation bars exhibit a shape in which a mass moment of inertia of a liquid quantity, which the oscillation bars move with them in the immersed state in the liquid, is as large as possible and greater than 0.2 times a mass moment of inertia of the oscillation bars.

TECHNICAL FIELD

The invention relates to a device for establishing and/or monitoring apredetermined fill level in a container.

BACKGROUND OF THE INVENTION

Fill level limit switches of this type are applied in many branches ofindustry, especially in the chemical and food industries. They serve fordetecting a limit level and are e.g. used as overfill guards orprotection against pumps running empty.

DE-A 44 19 617 describes a device for establishing and/or monitoring apredetermined fill level in a container. The device includes:

a mechanical oscillatory structure placed at the height of thepredetermined fill level,

which exhibits a membrane, or diaphragm, and two mutually separatedoscillation bars formed thereon,

an electromechanical transducer,

which in operation excites the oscillatory structure to oscillate withoscillations, such that the oscillation bars execute oscillationsperpendicular to their longitudinal axis, and

a receiver- and evaluation-unit, which uses the oscillation to establishand/or monitor whether the predetermined fill level has been reached, ornot.

The oscillation bars have flat, mutually parallel paddles on theirmembrane-far ends. A normal to the paddle surfaces is perpendicular tothe longitudinal axis of the paddles.

The electromechanical transducer has at least one transmitter, at whichan electrical, transmitted signal is applied and which excites themechanical oscillatory structure to oscillate. A receiver is provided,which picks up the mechanical oscillations of the oscillatory structureand transforms such into an electrical, received signal. The evaluationunit obtains the received signal and compares its frequency with areference frequency. It produces an output signal, which indicates thatthe mechanical oscillatory structure is covered by a fill material, ifthe frequency has a value smaller than a reference frequency, and thatit is not covered, if the value is larger. A control circuit isprovided, which regulates a phase difference between the electrical,transmitted signal and the electrical, received signal to a determined,constant value, at which the oscillatory structure executes oscillationswith a resonance frequency.

The control circuit is e.g. formed such that the received signal isamplified and fed back to the transmitted signal through a phaseshifter.

Such devices are used for a multiplicity of different applications andare, therefore, exposed to quite varied requirements.

SUMMARY OF THE INVENTION

An object of the invention is to provide a device for establishingand/or monitoring a predetermined fill level in a container, whichdevice has a best possible fit for a multiplicity of applications.

The object is solved, according to the invention, by a device forestablishing and/or monitoring a predetermined fill level in acontainer, which device includes:

a mechanical oscillatory structure placed at the height of thepredetermined fill level,

which exhibits a membrane and two mutually separated oscillation barsformed thereon,

an electromechanical transducer,

which in operation excites the oscillatory structure to oscillate withoscillations, such that the oscillation bars execute oscillationsperpendicular to their longitudinal axis, and

a receiver- and evaluation-unit, which uses the oscillations toestablish and/or monitor whether the predetermined fill level has beenreached, or not,

in which the oscillation bars exhibit a shape in which a mass moment ofinertia of a liquid quantity, which the oscillation bars move with themin the immersed state in the liquid, is as large as possible and greaterthan 0.2 times a mass moment of inertia of the oscillation bars.

According to one embodiment, the oscillation bars have flat, mutuallyparallel paddles on their membrane-far ends, wherein a normal to thepaddle surfaces is perpendicular to the longitudinal axis of theoscillation bars.

According to a further development

the oscillation bars extend in operation through an opening into thecontainer,

the opening has a diameter of less than five centimeters,

the membrane has a diameter which is slightly smaller than the diameterof the opening,

the paddles have a maximum width, wherein an outer diameter of thedevice in the region of the oscillation bars is smaller than, or equalto, the diameter of the opening.

According to a further development, a length L of the oscillation barsincluding the paddles is chosen such that a resonance frequency of theoscillatory structure is smaller than 1400 Hz at maximum paddle width.

According to a further development, the paddles have a length l, whichamounts to 50%+/−10% of the length L of the oscillation bars.

According to a further development, the paddles have a small thickness.

According to one embodiment, the membrane is made of a metal and has athickness of 0.6 to 1 mm.

According to a first embodiment,

the opening has a diameter of about 24 mm (½ inch),

the membrane is placed in the opening and closes it,

each oscillation bar has a mass moment of inertia in the range from lessthan, or equal to, 18 kgmm² to greater than, or equal to, 1.1 kgmm²,

the paddles have a thickness between 1 mm and 4.1 mm, and

the oscillation bars have a length between 37 mm and 60 mm.

According to a second embodiment,

the opening has a diameter of about 12 mm (¼ inch),

the membrane is placed in the opening and closes it,

each oscillation bar has a mass moment of inertia in the range from lessthan, or equal to, 1.6 kgmm² to greater than, or equal to, 0.4 kgmm²,

the paddles have a thickness between 1 mm and 2 mm, and

the oscillation bars have a length between 30 mm and 40 mm.

The invention also resides in a method for manufacturing one of theabove-recited devices, in which

from a predetermined diameter of the opening in the container, themaximum diameter of the membrane is determined,

a mutual separation of the paddles and their thickness is fixed as afunction of the diameter of the membrane,

subsequently, for obtaining a high sensitivity of the device, themaximum possible width of the paddles is determined,

a minimum length of the oscillation bars is ascertained, from which aresonance frequency of the oscillatory structure is less than 1400 Hz,and

the oscillatory structure is manufactured using the forenamedspecifications.

The oscillatory structure, in operation, executes forced harmonicoscillations. Preferably, the device is driven in resonance, since,then, an amplitude of the oscillations is maximum. An immersion of theoscillatory structure in the liquid effects an additional damping of theresonance oscillation and leads to a reduction of the oscillationamplitude and the resonance frequency. The reason for the damping isthat a quantity of liquid moves with the oscillation bars, as a functionof the shape of the oscillation bars.

By constructing the oscillation bars such that the mass moment ofinertia of the water mass, which moves with the oscillation bars in theimmersed state, is as large as possible in comparison with the massmoment of inertia of the oscillation bars, the device exhibits a veryhigh sensitivity. I.e., a measurement effect resulting from theimmersion in the liquid is very large. For the mass moments of inertiadiscussed here, a reference axis lies in each case in the plane of themembrane and extends perpendicular to the normal to the paddle surfaces.

Experiments have shown that it is sufficient for most applications thatthe mass moment of inertia of the moved liquid mass be at least equal to0.2 times the mass moment of inertia of the oscillation bars. Thisassures that the device operates error free, even under very difficultconditions, e.g. in media with a low density. For the size of the liquidquantity which moves with the oscillation bars, an area projected in thedirection of movement of the oscillation bars is the ruling factor. Thelarger the projected area, the larger is the quantity of liquid whichmoves in accompaniment.

A measure for the sensitivity of the device is a change of the resonancefrequency. In the following, the sensitivity 6 means the differencebetween the resonance frequency ω_(f), with which the oscillatorystructure oscillates, when it is immersed in the liquid, and theresonance frequency ω₀, with which the oscillatory structure oscillatesout of the liquid.

Experiments have shown that the sensitivity δ is a function of the ratioV of the mass moment of inertia of the liquid mass moved with theoscillation bars in the immersed state and the mass moment of inertia ofthe oscillation bars. The relationship is:δ=1−(1/(1+V))^(1/2)  (1)

For a ratio V of 0.2, the sensitivity is already 16%. The formula givenin Equation (1) is graphically presented in FIG. 1.

The invention and additional advantages are explained in more detail onthe basis of the figures of the drawing, where an example of anembodiment is presented; equal elements are given the same referencesymbols in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sensitivity of a device as a function of the ratio V ofthe mass moment of inertia of the liquid quantity which moves with theoscillation bars, to the mass moment of inertia of the oscillation bars;

FIG. 2 shows a longitudinal cross section through a device forestablishing and/or monitoring a predetermined fill level;

FIG. 3 shows a side view of an oscillation bar;

FIG. 4 shows a dependence of the sensitivity of the device on paddlewidth;

FIG. 5 shows a dependence of the sensitivity of the device on paddlelength.

FIG. 6 shows a dependence of the sensitivity of the device on paddlethickness;

FIG. 7 shows an example for a shape of the oscillation bars; and

FIG. 8 shows a further example for a shape of the oscillation bars.

DETAILED DESCRIPTION

FIG. 2 shows a longitudinal cross section through a device of theinvention for establishing and/or monitoring a predetermined fill levelin a container. The device has a mechanical oscillatory structure forplacement at the height of the predetermined fill level.

The oscillatory structure includes an essentially cylindrical housing 1,which is closed off flush at the front by a circular membrane 3. Athread 5 is formed on the housing 1, so that the device can be screwedinto a container opening 6 arranged at the height of the predeterminedfill level. Other techniques of securement known to those skilled in theart, e.g. flanges formed on the housing 1, can likewise be used.

Two oscillation bars 7, likewise part of the oscillatory structure, areformed on the membrane 3 outside of the housing 1 and point into thecontainer. FIG. 3 shows a view of the oscillation bars 7. These are madeto oscillate perpendicular to their longitudinal axes by anelectromechanical transducer 9 arranged on membrane 3 inside the housing1. The electromechanical transducer 9 in this embodiment is adisk-shaped piezoelectric element, which is situated on the membrane 3and fixedly connected therewith. The piezoelectric element is e.g.glued, soldered or brazed on the membrane 3 and causes the membrane toundergo bending oscillations when operating. This bending motion in turncauses the oscillation bars 7 to oscillate perpendicularly to their longaxes.

The oscillatory structure when operating is excited to oscillate withresonance oscillations by an electronic circuit 11, and a receiver- andevaluation-unit 13 is provided for establishing and/or monitoring on thebasis of the oscillation whether the predetermined fill level has beenreached, or not. This happens, for example, by the arrangement on amembrane-far side of the piezoelectric element 9 of a transmittingelectrode 15 and a receiving electrode 17.

The electronic circuit 11 places on the transmitting electrode 15 anelectrical, transmitted signal, which excites the mechanical oscillatorystructure to oscillate. The oscillations are received by the receivingelectrode 17 and changed into an electrical, received signal. Thereceiver- and evaluation-unit 13 receives the received signal andcompares its frequency with a reference frequency. It produces an outputsignal which indicates that the mechanical oscillatory structure iscovered by a fill material, when the frequency has a value smaller thanthe reference frequency, and that it is not covered, when the value islarger. In the electrical circuit 11, a control circuit is provided,which regulates a phase difference between the electrical, transmittedsignal and the electrical, received signal to a determined, constantvalue, at which the oscillatory structure executes oscillations with aresonance frequency.

The control circuit is e.g. constructed such that the received signal isamplified and fed back through a phase shifter onto the transmittedsignal.

According to the invention, the oscillation bars exhibit a shape inwhich a mass moment of inertia of a liquid quantity, which theoscillation bars move with them in the immersed state in the liquid, isas large as possible and greater than 0.2 times a mass moment of inertiaof the oscillation bars.

“Mass moment of inertia” means always the mass moment of inertia of theoscillation bars 7, or of the liquid mass which moves with theoscillation bars, as the case may be, referenced to the axis in theplane of the membrane 3 and extending perpendicular to the normal to thesurfaces of the paddles 8.

Constructing the oscillation bars in this way assures that the devicehas a high sensitivity δ. A high sensitivity. δ is fundamental for anoptimum fitting of the device to a multiplicity of applications. On thebasis of the high sensitivity 6, the device can be used without problemeven in otherwise very difficult applications, e.g. in liquids with verylow density.

An increase of the ratio V, which, as already explained above, is thecritical parameter, can occur in many ways. A large ratio V is present,when the oscillation bars 7 exhibit a shape, in which a thickness of theliquid layer moved with the oscillation bars 7 is as large as possible.For this, an area of the oscillation bars 7 projected in the directionof movement is decisive. The greater the area moved through the liquidis in the projection, the greater is also the quantity of liquid thatmoves therewith.

Since flat elements are better suited for moving large quantities ofliquid with them, the oscillation bars 7 are equipped, as shown in FIG.3, preferably terminally on their membrane-far ends, with flat, mutuallyparallel paddles 8, which are oriented such that the normal to theirsurfaces extends perpendicular to the longitudinal axis of theoscillation bars 7.

Investigations have shown that the ratio of the mass moment of inertiaof the liquid moved with the oscillation bars referenced to the massmoment of inertia of the oscillation bars, so important for thesensitivity of the device, can be noticeably increased, by increasingthe width of the oscillation bars 7, or, in the case of the embodimentof the drawing, the width b of the paddle 8. FIG. 4 shows schematicallythe sensitivity δ as a function of the width b of the paddles 8. Alengthening of the oscillation bars 7 and/or the paddles 8, in contrast,does not lead to a noticeable increase of the ratio V of the two massmoments of inertia. A lengthening of the oscillation bars 7 increasesthe mass moment of inertia of the moved liquid and the mass moment ofinertia about to the same extent. Consequently, the width b of thepaddles 8 is to be maximized, in order to achieve a high sensitivity ofthe device.

The oscillation bars 7 protrude in operation through the opening 6 intothe container. The opening 6 has a diameter of a few centimeters.Membrane 3 has a diameter that is slightly less than the diameter of theopening 6. Correspondingly, the paddles 8 preferably have a maximumwidth b, at which an outer diameter of the device in the region of theoscillation bars is slightly less than the diameter of the opening 6 inthe container.

Although increasing the width b and the length l of the paddles bothincrease the mass moment of inertia of the oscillation bars 7, onlyincreasing the width b leads to a noticeable increase in the sensitivityδ of the device.

To a limited extent, decreasing the thickness d of the paddles canincrease the ratio V. Thinner paddles 8 have, for equal projected areamoved against the liquid, a smaller mass, and, consequently, a smallermass moment of inertial than otherwise identical oscillation bars withthicker paddles 8. Since the projected area moved through the liquidremains the same, the amount of liquid moved also remains the same and,consequently, also its mass moment of inertia. Correspondingly, theratio V of the mass moments of inertia increases. FIG. 6 shown thedependence of the sensitivity of the device on paddle thickness d.

Naturally, decreasing the thickness d of the paddles 8 has limits, whichcome from the requirement that the oscillation bars 7 and their paddles8 must not deform, bend or break under the mechanical loads associatedwith an application. In the case of metallic oscillation bars 7, forreasons of mechanical stability, thicknesses below a limit value of onemillimeter thickness should not be used.

The mass moment of inertia of the oscillation bars 7 can, depending onthe shape of the oscillation bars, be determined either directly, bymeans of approximation calculations, or by simulation calculations, e.g.by means of methods of finite element analysis. The mass moment ofinertia of the moved liquid mass can be determined indirectly fromEquation (1). For this, the sensitivity δ of the device must becalculated in a first step experimentally or numerically and the massmoment of inertia of the oscillation bars 7 must be available.Simulation programs are available today for the numerical calculation ofthe sensitivity δ, programs such as e.g. the software package ANSYS ofthe firm ANSYS, Inc., Canonsburg, Pa. 15317, with which an immersion ofthe oscillation bars 7 in a liquid can be simulated, and from thesimulations the oscillation frequencies can be determined.

Since a length L of the oscillation bars 7 has essentially no influenceon the ratio V of the mass moments of inertia, but does indeed affectthe mass moment of inertia of the oscillation bars 7, the length L ofthe oscillation bars 7 can be used to set a desired resonance frequency.Preferably, the length L of the oscillation bars 7, including thepaddles 8, is chosen such that the resonance frequency of theoscillatory structure for maximum width b of the paddles is smaller than1400 Hz. This assures that the device will still work reliably even ineffervescing media, e.g. in water containing carbon dioxide.

Preferably, the paddles 8 have a length l, which is 50%+/−10% of thelength L of the oscillation bars. A further increasing of the length lof the paddles 8 with respect to the length L of the oscillation bars 7brings only very little increase in the sensitivity δ, less than 5%, sothat it would mean extra cost for material, without a correspondingpayback for most applications.

The membrane 3 is made of metal and has a thickness of 0.6 to 1 mm. Sucha thickness provides for a metallic membrane 3 an adequate factor ofsafety, such that the membrane 3 can withstand even strong loads, e.g.from high pressures or mechanical stress.

The following are two optimized examples for oscillation bars 7 withpaddles 8.

For a container, in which the opening has a diameter of about 24 mm (½inch), and the membrane 3 is mounted in the opening in such a way as toclose the opening, the oscillation bars 7 preferably have a mass momentof inertia that is less than, or equal to, 18 kgmm² and greater than, orequal to, 1.1 kgmm². The paddles have in such case preferably athickness between 1 mm and 4.1 mm, and the oscillation bars 7 have alength between 37 mm and 60 mm.

For a container, in which the opening has a diameter of about 12 mm (¼inch), and the membrane 3 is mounted in the opening in such a way as toclose the opening, the oscillation bars 7 preferably have a mass momentof inertia that is less than, or equal to, 1.6 kgmm² and greater than,or equal to, 0.4 kgmm². The paddles have in such case preferably athickness between 1 mm and 2 mm, and the oscillation bars 7 have alength between 30 mm and 40 mm.

In order to achieve an optimum design for an application, the device ismanufactured as follows: First, the maximum diameter of the membrane 3is determined for a given diameter of the opening 6. Then a mutualseparation of the oscillation bars 7 and their thickness is chosen as afunction of the diameter of the membrane 3. Next, to obtain a highsensitivity δ for the device, the maximum possible width b of thepaddles 8 is determined. This follows from the constraint that it muststill be possible to insert the paddles 8 through the opening into thecontainer. Finally, a minimum length L of the oscillation bars 7 isestablished, from which a resonance frequency of the oscillatorystructure is less than 1400 Hz. The oscillatory structure is then built,taking into consideration the dimensional data determined as described.

A shaping of the paddles 8 and its effects on sensitivity can beestablished numerically. This is explained in the following on the basisof two special shapes. A first shape A is presented in FIG. 7. This isthe simplest case of an oscillation bar 7 having a bar of rectangularcross section of width bs and a paddle 8 formed on its membrane-far end.Paddle 8 is likewise rectangular in cross section, with a width b and alength l. The length l of the paddle 8 is 0.5 times the total length Lof the oscillation bar 7.

FIG. 8 shows an alternative shape B for an oscillation bar 7. Thisoscillation bar 7 likewise exhibits a bar of rectangular cross sectionof width bs, with a paddle 8 formed on its membrane-far end. The paddle8 is rectangular in cross section with a tip 17 pointing away from themembrane. Tip 17 converges with both sides inclined at angles of 45° tothe longitudinal axis of the oscillation bar and terminates with atruncated end of width sp. The paddle 8 itself again has the overalldimensions of width b and length l. The length l of the paddle 8 is 0.5times the total length L of the oscillation bar 7.

The above mentioned program Ansys provides prefabricated liquid elementsfor simulation of an oscillation of the oscillation bar 7 in a liquid.Consequently, this program can be used to model an oscillation of theoscillation bar 7 in the air and in a liquid. This gives the resonancefrequency for both situations, and, with that, as explained above, thesensitivity δ of the device can be determined.

For a bar width bs of 3 mm in the case of Shape A, the following formulais obtained for the sensitivity:

$\begin{matrix}{{\delta(d)}:={\left\lbrack {1 - \sqrt{\frac{1}{1 + \frac{\left( {{0,{0121 \cdot b^{2}}} + {0,{026 \cdot b}}} \right) \cdot \left( \frac{1}{40} \right)^{3} \cdot \left( \frac{pfl}{10^{- 6}} \right)}{\frac{1}{24} \cdot d \cdot {pp} \cdot {.\left( {3 + {7 \cdot b}} \right)}}}}} \right\rbrack \cdot 100}} & {{Form}\mspace{14mu} A}\end{matrix}$where ρfl is the density of the liquid and ρp is the density of theoscillation bar 7, the densities are in kg/mm³ and the lengths are inmm.

For the Shape B with a bar width bs of 3 mm and a width sp of 1 mm forthe truncated end, the following formula is obtained for thesensitivity:

$\begin{matrix}{{\delta(d)}:={\left\lbrack {1 - \sqrt{\frac{1}{1 + \frac{\left( {{0,{01 \cdot b^{2}}} + {0,{043 \cdot b}}} \right) \cdot \left\lbrack {\left( \frac{\lg}{40} \right)^{3,33} \cdot \left( \frac{pfl}{p\; H_{2}O} \right)} \right\rbrack}{{{\frac{1}{24} \cdot d \cdot \lg^{3}}{{pp} \cdot \left( {3 + {7 \cdot b \cdot p}} \right)}} - {\frac{1}{192} \cdot \left( {b - 1} \right)^{4} \cdot d \cdot {pp} \cdot \left\lbrack {1 + \frac{\left\lbrack {\lg - {\frac{1}{6} \cdot \left( {b - 1} \right)^{2}}} \right\rbrack}{\left( {b - 1} \right)^{2}}} \right\rbrack}}}}} \right\rbrack \cdot 100}} & {{Form}\mspace{14mu} B}\end{matrix}$

Of course, complicated shapes can also be evaluated numerically, and thedependence of sensitivity δ on other parameters can be calculated. Thisleads to an evaluation of the ratio V for any particular shape. Thelarger the projected area of the special paddle shape with the leastpossible mass moment of inertia of the oscillation bar 7, the greater isthe sensitivity δ of the device.

1. A device for establishing and/or monitoring a predetermined filllevel in a container, comprising: a mechanical oscillatory structuremounted to the container at the height of the predetermined fill level,said mechanical oscillatory structure having a membrane and two mutuallyseparated oscillation bars formed thereon; an electromechanicaltransducer associate with and adapted to excite said mechanicaloscillatory structure to oscillate with oscillations such that saidoscillation bars execute oscillations perpendicular to theirlongitudinal axis; and a receiver and evaluation unit connected to saidelectromechanical transducer, which uses the oscillations to establishand/or monitor whether the predetermined fill level has been reached, ornot, wherein: said oscillation bars are immersed in and moved in aliquid quantity in the container; and said oscillation bars have a shapein which a mass moment of inertia of a liquid quantity is as large aspossible and greater than 0.2 times the mass moment of inertia of saidoscillation bars.
 2. The device as defined in claim 1, wherein: saidoscillation bars define flat, mutually parallel paddles on theirmembrane-free ends; and a normal axis to the surface of said paddlesextends perpendicular to the longitudinal axis of said oscillation bars.3. The device as defined in claim 2, wherein: the container has anopening through which said oscillation bars extend, the opening having adiameter of less than 5 cm; said membrane has a diameter slightlysmaller than the diameter of the opening; and said paddles have amaximum width, where an outer diameter of the device in the region ofsaid oscillation bars is smaller than, or equal to, the diameter of theopening.
 4. The device as defined in claim 3, wherein: the length ofsaid oscillation bars, including the paddles, is such that a resonancefrequency of said oscillatory structure is smaller than 1400 Hz at amaximum width of said paddles.
 5. The device as defined in claim 3,wherein: said paddles have a length which amounts to 50%+/−10% of thelength of said oscillation bars.
 6. The device as defined in claim 2,wherein: said paddles have a small thickness.
 7. The device as definedin claim 2, wherein: said membrane is made of metal and has a thicknessof 0.6 to 1 mm.
 8. The device as defined in claim 2, wherein: thecontainer has an opening through which said oscillation bars extend, theopening having a diameter of less than 5 cm; said membrane has adiameter slightly smaller than the diameter of the opening; and saidpaddles have a maximum width, where an outer diameter of the device inthe region of said oscillation bars is smaller than, or equal to, thediameter of the opening, wherein: the opening has a diameter of about 24mm, said membrane is placed in and closes the opening; each oscillationbar has a mass movement of inertia in the range from less than, or equalto, 18 kgmm² to greater than, or equal to 1.1. kgmm²; said paddles havea thickness between 1 mm and 4.1 mm; and said oscillation bars have alength between 37 mm and 60 mm.
 9. The device as defined in claim 2,wherein: the container has an opening through which said oscillationbars extend, the opening having a diameter of less than 5 cm; saidmembrane has a diameter slightly smaller than the diameter of theopening; and said paddles have a maximum width, where an outer diameterof the device in the region of said oscillation bars is smaller than, orequal to, the diameter of the opening, wherein: the opening has adiameter of about 12 mm, said membrane is placed in and closes theopening; each oscillation bar has a mass moment of inertia in the rangefrom less than, or equal to, 1.6 kgmm² to greater than, or equal to, 0.4kgmm²; said paddles have a thickness between 1 mm and 2 mm; and saidoscillation bars have a length between 30 mm and 40 mm.
 10. A method ofmanufacturing a device for a container, including: a mechanicaloscillatory structure having a membrane and two mutually separatedoscillation bars; an electromechanical transducer; and a receiver andevaluation unit, the method comprising the steps of: determining themaximum diameter of the membrane from the predetermined diameter of theopening in the container; fixing the mutual separation of the paddlesand their thickness as a function of the diameter of the membrane;subsequently determining the maximum possible width of the paddles forobtaining a high sensitivity of the device; and ascertaining a minimumlength of the oscillation bars, from which a resonance frequency of theoscillatory structure is less than 1400 Hz.